MAXM17225 datasheet(12/14 Pages) MAXIM

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Tiny, 0.4V to 5.5V Input, 300nA IQ, nanoPower

Boost Module with True Shutdown

MAXM17225

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VIN > VOUT, VIN ≈ VOUT Operation

If the input voltage (VIN) is greater than the output voltage (VOUT) by a diode drop (VDIODE varies from ~0.2V at light load

to ~0.7V at heavy load), then the output voltage is clamped to a diode drop below the input voltage (i.e., VOUT = VIN -

VDIODE). When the input voltage is closer to the output voltage target (i.e., VOUT TARGET + VDIODE > VIN > VOUT TARGET),

the MAXM17225 operates like a buck converter with higher output voltage ripple.

In boost mode, if VIN and VOUT separation is 250mV or lesser, it is suggested to have a higher output capacitance than

the recommended 1x10µF to reduce the output voltage ripple. The exact capacitance value depends on the application

need and acceptable voltage ripple in the application. In most cases, 2x10µF will be more than sufficient.

Soft-Start

The MAXM17225 initiates a controlled soft-start in the event that a supply voltage is reapplied at a high dV/dt rate, a

typical example is during the installation of a fresh battery. While in regulation, if VIN steps abruptly above VOUT for more

than 1V (typ), the device resets. The output voltage droop, in this case, will be a function of the load current, output

capacitance, and time required for soft-start to complete, which is 1.5ms (typ). The IC has 3 slew rate routines, which the

internal advanced algorithm auto-chooses based on the converter state, those are linear ramp of VOUT with time called

linear slew up, the second one is open loop low voltage oscillation (this is uncontrolled), and the last one is the linear

slewing where the synchronous rectifier acts as a current source controlling the ramp up.

Applications Information

Input Capacitor

The input capacitor (CIN) reduces the peak current drawn from battery or input power source and reduces the switching

noise in the module. The impedance of CIN at the switching frequency should be very low. Ceramic capacitors are

recommended for their small size and low ESR. For most applications, use a 10µF ceramic capacitor with X7R

temperature characteristics, making it suitable for 125°C module operation. While choosing capacitor dielectric other than

X7R, keep a check on % capacitance change across temperature. It is also recommended to check the DC bias curves

to determine the minimum effective capacitance at the application voltage for the rated capacitance value chosen. X5R

capacitor can also be used if the worst-case capacitor surface temperature in the product at worst-case operating

temperature is well below 85°C.

Output Capacitor

The output capacitor (COUT) is required to keep the output voltage ripple small and to ensure loop stability. COUT must

have low impedance at the switching frequency. Ceramic capacitors are recommended due to their small size and low

ESR. Make sure the capacitor does not degrade its capacitance significantly over temperature and DC bias. Capacitors

with X7R temperature characteristics typically perform well. X5R can also be chosen if the worst-case capacitor surface

temperature is well below 85°C. For most applications, it is recommended to use a 10µF X7R ceramic output capacitor.

Enable Input

The MAXM17225 has a separate enable pin to enable/disable the device. The typical falling voltage threshold for enable

is 0.5V, and likewise, the rising voltage threshold at device startup is 0.6V at room temp. When EN and IN are tied

together, the enable threshold of the device can interfere with product operation as VIN drops below 550mV, preventing

the device from reaching its minimum specification (i.e., VIN_MIN) of 400mV. In such cases, it is recommended to use an

external enable, independent of VIN and external VEN level must be higher than device VEN threshold to ensure the device

operates as low as 400mV VIN, post startup.

RSEL Considerations

The single RSEL external resistor used for VOUT selection needs some considerations. The trace parasitic capacitance

from RSEL pin to external resistor should be less than 2pF to facilitate an accurate resistance read at device startup. The

typical resistor read time is 600µs at device startup, and this happens only once. A minimum of 1.8V at VOUT is needed

for internal ADC to accurately read the resistance value and configure the VOUT.

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